Function extract_usage 

pub fn extract_usage(root: &Value) -> AutomationUsage

Extract token usage from an OpenAI-compatible response.

The response format follows the OpenAI API structure:

{
  "usage": {
    "prompt_tokens": 123,
    "completion_tokens": 456,
    "total_tokens": 579
  }
}

Returns a default AutomationUsage if the usage field is missing or malformed.